Membrane-based processes for removing water from water/organic compound mixtures are known. The processes may use membranes that are selective in favor of water over organics, or selective for organics over water. Most processes use water-selective membranes. Such membranes typically have a dense, hydrophilic water-selective layer on a porous support, and are very effective at treating solutions or mixtures in which water is present in relatively small amounts.
Representative membranes of this type are described in U.S. Pat. No. 4,755,299, for example. Typical materials from which the selective layer of the membrane are made include polyvinyl alcohol (PVA), cellulose triacetate and other cellulose derivatives. If large amounts of water are present, this selective layer tends to swell, reducing the separation capability of the membrane. Under prolonged exposure to high water concentrations, the membrane may start to dissolve or disintegrate completely. The problem is exacerbated if the feed solution is hot. Such membranes cannot be used to treat fluids that have high water concentrations.
In a variety of industrial applications, most notably manufacture of alcohols from biomass, the solutions to be treated are hot and can contain 20, 30, 40, 50 wt % or more of water. To treat these and other similar solutions or vapor mixtures, there remains a need for membranes that are able to withstand such conditions.
U.S. Published Patent Application 2007/0031954, co-owned with the present application, describes an ethanol recovery process using both ethanol-selective and water-selective membranes.
Currently pending U.S. patent application Ser. No. 11/715,245, filed Mar. 6, 2007, entitled Liquid-Phase and Vapor-Phase Dehydration of Organic/Water Solutions, and co-owned with the present application, describes the use of fluorinated dioxole membranes to dehydrate alcohols and other organic compounds. This patent application is incorporated herein by reference in its entirety.
U.S. Pat. No. 6,896,717 describes gas separation membranes having hydrocarbon-resistant coatings, include those made from perfluorinated polymers of various types, to protect them from attack by C3+ hydrocarbons in the feed stream.